This invention relates to a stop flow interference lithography system and more particularly to such a system for the high throughput synthesis of 3-dimensionally patterned polymeric particles.
Polymeric structures with repeating 2-dimensional (2D) and 3D motifs at the micron scale and below have a variety of uses. Patterned 2D structures have been shown to have myriad applications in biosensors,[1] tissue engineering,[2] and diagnostic assay systems.[3] The numbers in brackets refer to the references included herein. The contents of all of these references are incorporated herein by reference. The availability of more sophisticated 3D structures would enable important advances in photonics,[4] information storage,[5] and tissue engineering.[6,7] Many techniques have been developed to fabricate complex 3D structures at the micron scale and below using either top-down or bottom-up approaches. Bottom-up approaches like polymer phase separation,[8] molecular self-assembly,[9] or colloidal assembly[10] are cheap and can cover large areas but face problems of defects and limitations in the type and the geometry of structures that can be formed. While top-down methods offer precise size and shape control, the need to construct the 3D structures using either a point-by-point or layer-by-layer process makes such methods as gray-scale photolithography,[11] direct 3D writing,[12] or 2-photon lithography[13] very time consuming.
Recently, Interference Lithography (IL)[14-20] has emerged as an attractive alternative technique, that allows one to rationally design complex and defect-free 1D, 2D and 3D patterns over large areas. Besides being fast and efficient, IL also affords control over geometrical parameters, such as symmetry and volume fractions, of the structures formed. IL performed using an elastomeric PDMS phase mask[17] has the further advantage of providing simple and inexpensive processing, since only a single collimated beam is needed to form 3D interference patterns. However, phase mask interference lithography (PMIL)[16,17] is typically performed by flood exposing a spin-coated layer of photoresist film through a phase mask, thus imposing some restrictions on the shape and material properties of the structures formed as well as limiting the throughput because of the serial nature of the processing. Furthermore, such processes are not easily amenable to the fabrication of structures that possess chemical anisotropy. Chemical anisotropy refers to the presence of multiple, segregated chemical functionalities or gradients of one or more chemical functionality in a structure. The synthesis of structures that possess controllable material properties and texturing across multiple length scales is important for a variety of applications such as tissue engineering[2], self-assembly[21] or particle diagnostics.[3] To address this challenge, several microfluidic techniques that combine traditional photopolymerization or lithography with the unique properties of flow at the micron scale have emerged recently.[22,23] The Doyle group developed a simple, flow-through microfluidic process called Stop Flow Lithography (SFL)[24] that enables photolithography to be performed in a flowing stream of oligomer. This enables the high throughput synthesis of large numbers of micron-sized particles in any 2D extruded shape using a variety of polymer precursors.[24] The method also provides the ability to finely and conveniently tune the chemical anisotropy of the structures formed. However, to date, the use of transparency masks has restricted the method to the formation of only solid 2D shapes with relatively large feature sizes.[22,25]